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Cruz AF, Herrmann J, Ramcharran H, Kollisch-Singule M, Tawhai MH, Bates JHT, Nieman GF, Kaczka DW. Sustained vs. Intratidal Recruitment in the Injured Lung During Airway Pressure Release Ventilation: A Computational Modeling Perspective. Mil Med 2023; 188:141-148. [PMID: 37948236 DOI: 10.1093/milmed/usad059] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2022] [Revised: 02/03/2023] [Accepted: 02/14/2023] [Indexed: 11/12/2023] Open
Abstract
INTRODUCTION During mechanical ventilation, cyclic recruitment and derecruitment (R/D) of alveoli result in focal points of heterogeneous stress throughout the lung. In the acutely injured lung, the rates at which alveoli can be recruited or derecruited may also be altered, requiring longer times at higher pressure levels to be recruited during inspiration, but shorter times at lower pressure levels to minimize collapse during exhalation. In this study, we used a computational model to simulate the effects of airway pressure release ventilation (APRV) on acinar recruitment, with varying inspiratory pressure levels and durations of exhalation. MATERIALS AND METHODS The computational model consisted of a ventilator pressure source, a distensible breathing circuit, an endotracheal tube, and a porcine lung consisting of recruited and derecruited zones, as well as a transitional zone capable of intratidal R/D. Lung injury was simulated by modifying each acinus with an inflation-dependent surface tension. APRV was simulated for an inhalation duration (Thigh) of 4.0 seconds, inspiratory pressures (Phigh) of 28 and 40 cmH2O, and exhalation durations (Tlow) ranging from 0.2 to 1.5 seconds. RESULTS Both sustained acinar recruitment and intratidal R/D within the subtree were consistently higher for Phigh of 40 cmH2O vs. 28 cmH2O, regardless of Tlow. Increasing Tlow was associated with decreasing sustained acinar recruitment, but increasing intratidal R/D, within the subtree. Increasing Tlow was associated with decreasing elastance of both the total respiratory system and transitional subtree of the model. CONCLUSIONS Our computational model demonstrates the confounding effects of cyclic R/D, sustained recruitment, and parenchymal strain stiffening on estimates of total lung elastance during APRV. Increasing inspiratory pressures leads to not only more sustained recruitment of unstable acini but also more intratidal R/D. Our model indicates that higher inspiratory pressures should be used in conjunction with shorter exhalation times, to avoid increasing intratidal R/D.
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Affiliation(s)
- Andrea F Cruz
- Department of Anesthesia, University of Iowa, Iowa City, IA 52242, USA
| | - Jacob Herrmann
- Roy J. Carver Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242, USA
| | - Harry Ramcharran
- Department of Surgery, SUNY Upstate Medical Center, Syracuse, NY 13210, USA
| | | | - Merryn H Tawhai
- Department of Bioengineering, University of Auckland, Auckland 1124, New Zealand
| | - Jason H T Bates
- Department of Medicine, University of Vermont, Burlington, VT 05405, USA
| | - Gary F Nieman
- Department of Surgery, SUNY Upstate Medical Center, Syracuse, NY 13210, USA
| | - David W Kaczka
- Department of Anesthesia, University of Iowa, Iowa City, IA 52242, USA
- Roy J. Carver Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242, USA
- Department of Radiology, University of Iowa, Iowa City, IA 52242, USA
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Sattari S, Mariano CA, Eskandari M. Pressure-volume mechanics of inflating and deflating intact whole organ porcine lungs. J Biomech 2023; 157:111696. [PMID: 37413822 DOI: 10.1016/j.jbiomech.2023.111696] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2022] [Revised: 06/14/2023] [Accepted: 06/19/2023] [Indexed: 07/08/2023]
Abstract
Pressure-volume curves of the lung are classical measurements of lung function and are impacted by changes in lung structure due to disease or shifts in air-delivery volume or cycling rate. Diseased and preterm infant lungs have been found to show heterogeneous behavior which is highly frequency dependent. This breathing rate dependency has motivated the exploration of multi-frequency oscillatory ventilators to deliver volume oscillation with optimal frequencies for various portions of the lung to provide more uniform air distribution. The design of these advanced ventilators requires the examination of lung function and mechanics, and an improved understanding of the pressure-volume response of the lung. Therefore, to comprehensively analyze whole lung organ mechanics, we investigate six combinations of varying applied volumes and frequencies using ex-vivo porcine specimens and our custom-designed electromechanical breathing apparatus. Lung responses were evaluated through measurements of inflation and deflation slopes, static compliance, peak pressure and volume, as well as hysteresis, energy loss, and pressure relaxation. Generally, we observed that the lungs were stiffer when subjected to faster breathing rates and lower inflation volumes. The lungs exhibited greater inflation volume dependencies compared to frequency dependencies. This study's reported response of the lung to variations of inflation volume and breathing rate can help the optimization of conventional mechanical ventilators and inform the design of advanced ventilators. Although frequency dependency is found to be minimal in normal porcine lungs, this preliminary study lays a foundation for comparison with pathological lungs, which are known to demonstrate marked rate dependency.
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Affiliation(s)
- Samaneh Sattari
- Department of Mechanical Engineering, University of California at Riverside, Riverside, CA, USA
| | - Crystal A Mariano
- Department of Mechanical Engineering, University of California at Riverside, Riverside, CA, USA
| | - Mona Eskandari
- Department of Mechanical Engineering, University of California at Riverside, Riverside, CA, USA; BREATHE Center, School of Medicine, University of California at Riverside, Riverside, CA, USA; Department of Bioengineering, University of California at Riverside, Riverside, CA, USA.
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Herrmann J, Kollisch-Singule M, Satalin J, Nieman GF, Kaczka DW. Assessment of Heterogeneity in Lung Structure and Function During Mechanical Ventilation: A Review of Methodologies. JOURNAL OF ENGINEERING AND SCIENCE IN MEDICAL DIAGNOSTICS AND THERAPY 2022; 5:040801. [PMID: 35832339 PMCID: PMC9132008 DOI: 10.1115/1.4054386] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/23/2021] [Revised: 04/13/2022] [Indexed: 06/15/2023]
Abstract
The mammalian lung is characterized by heterogeneity in both its structure and function, by incorporating an asymmetric branching airway tree optimized for maintenance of efficient ventilation, perfusion, and gas exchange. Despite potential benefits of naturally occurring heterogeneity in the lungs, there may also be detrimental effects arising from pathologic processes, which may result in deficiencies in gas transport and exchange. Regardless of etiology, pathologic heterogeneity results in the maldistribution of regional ventilation and perfusion, impairments in gas exchange, and increased work of breathing. In extreme situations, heterogeneity may result in respiratory failure, necessitating support with a mechanical ventilator. This review will present a summary of measurement techniques for assessing and quantifying heterogeneity in respiratory system structure and function during mechanical ventilation. These methods have been grouped according to four broad categories: (1) inverse modeling of heterogeneous mechanical function; (2) capnography and washout techniques to measure heterogeneity of gas transport; (3) measurements of heterogeneous deformation on the surface of the lung; and finally (4) imaging techniques used to observe spatially-distributed ventilation or regional deformation. Each technique varies with regard to spatial and temporal resolution, degrees of invasiveness, risks posed to patients, as well as suitability for clinical implementation. Nonetheless, each technique provides a unique perspective on the manifestations and consequences of mechanical heterogeneity in the diseased lung.
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Affiliation(s)
- Jacob Herrmann
- Roy J. Carver Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242
| | | | - Joshua Satalin
- Department of Surgery, SUNY Upstate Medical University, Syracuse, NY 13210
| | - Gary F. Nieman
- Department of Surgery, SUNY Upstate Medical University, Syracuse, NY 13210
| | - David W. Kaczka
- Roy J. Carver Department of Biomedical Engineering, University of Iowa, Iowa City, IA 52242; Department of Anesthesia, University of Iowa, Iowa City, IA 52242; Department of Radiology, University of Iowa, Iowa City, IA 52242
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Yuki K, Koutsogiannaki S. Translational Role of Rodent Models to Study Ventilator-Induced Lung Injury. TRANSLATIONAL PERIOPERATIVE AND PAIN MEDICINE 2021; 8:404-415. [PMID: 34993270 PMCID: PMC8729883] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Mechanical ventilation is an important part of medical care in intensive care units and operating rooms to support respiration. While it is a critical component of medical care, it is well known that mechanical ventilation itself can be injurious to the lungs. Despite a large number of clinical and preclinical studies that have been done so far, there still exists a gap of knowledge regarding how to ventilate patients mechanically without increasing lung injury. Here, we will review what we have learned so far from preclinical and clinical studies and consider how to use preclinical models of ventilation-induced lung injury that better recapitulate the clinical scenarios.
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Affiliation(s)
- Koichi Yuki
- Cardiac Anesthesia Division, Department of Anesthesiology, Critical Care and Pain Medicine, Boston Children’s Hospital, USA,Department of Anaesthesia, Harvard Medical School, USA,Corresponding Authors: Sophia Koutsogiannaki, Ph.D and Koichi Yuki, M.D., Department of Anesthesiology, Critical Care and Pain Medicine, Cardiac Anesthesia Division, Boston Children’s Hospital, USA, ;
| | - Sophia Koutsogiannaki
- Cardiac Anesthesia Division, Department of Anesthesiology, Critical Care and Pain Medicine, Boston Children’s Hospital, USA,Department of Anaesthesia, Harvard Medical School, USA,Corresponding Authors: Sophia Koutsogiannaki, Ph.D and Koichi Yuki, M.D., Department of Anesthesiology, Critical Care and Pain Medicine, Cardiac Anesthesia Division, Boston Children’s Hospital, USA, ;
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Kaczka DW. Oscillatory ventilation redux: alternative perspectives on ventilator-induced lung injury in the acute respiratory distress syndrome. CURRENT OPINION IN PHYSIOLOGY 2021; 21:36-43. [PMID: 33898903 PMCID: PMC8056876 DOI: 10.1016/j.cophys.2021.03.006] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
For patients with the acute respiratory distress syndrome (ARDS), ventilation strategies that limit end-expiratory derecruitment and end-inspiratory overdistension are the only interventions to have significantly reduced the morbidity and mortality. For this reason, the use of high-frequency oscillatory ventilation (HFOV) was considered to be an ideal protective strategy, given its reliance on very low tidal volumes cycled at very high rates. However, results from clinical trials in adults with ARDS have demonstrated that HFOV does not improve clinical outcomes. Recent experimental and computational studies have shown that oscillation of a mechanically heterogeneous lung with multiple simultaneous frequencies can reduce parenchymal strain, improve gas exchange, and maintain lung recruitment at lower distending pressures compared to traditional ‘single-frequency’ HFOV. This review will discuss the theoretical rationale for the use of multiple oscillatory frequencies in ARDS, as well as the mechanisms by which it may reduce the risk for ventilator-induced lung injury.
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Affiliation(s)
- David W Kaczka
- Department of Anesthesia, University of Iowa, Iowa City, IA, USA.,Roy J. Carver Department of Biomedical Engineering, University of Iowa, Iowa City, IA, USA.,Department of Radiology, University of Iowa, Iowa City, IA, USA
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High-Frequency Oscillatory Ventilation and Ventilator-Induced Lung Injury: Size Does Matter. Crit Care Med 2020; 48:e66-e73. [PMID: 31634232 DOI: 10.1097/ccm.0000000000004073] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
OBJECTIVES The theoretical basis for minimizing tidal volume during high-frequency oscillatory ventilation may not be appropriate when lung tissue stretch occurs heterogeneously and/or rapidly. The objective of this study was to assess the extent to which increased ventilation heterogeneity may contribute to ventilator-induced lung injury during high-frequency oscillatory ventilation in adults compared with neonates on the basis of lung size, using a computational model of human lungs. DESIGN Computational modeling study. SETTING Research laboratory. SUBJECTS High-fidelity, 3D computational models of human lungs, scaled to various sizes representative of neonates, children, and adults, with varying injury severity. All models were generated from one thoracic CT image of a healthy adult male. INTERVENTIONS Oscillatory ventilation was simulated in each lung model at frequencies ranging from 0.2 to 40 Hz. Sinusoidal flow oscillations were delivered at the airway opening of each model and distributed through the lungs according to regional parenchymal mechanics. MEASUREMENTS AND MAIN RESULTS Acinar flow heterogeneity was assessed by the coefficient of variation in flow magnitudes across all acini in each model. High-frequency oscillatory ventilation simulations demonstrated increasing heterogeneity of regional parenchymal flow with increasing lung size, with decreasing ratio of deadspace to total acinar volume, and with increasing frequency above lung corner frequency and resonant frequency. Potential for resonant amplification was greatest in injured adult-sized lungs with higher regional quality factors indicating the presence of underdamped lung regions. CONCLUSIONS The potential for ventilator-induced lung injury during high-frequency oscillatory ventilation is enhanced at frequencies above lung corner frequency or resonant frequency despite reduced tidal volumes, especially in adults, due to regional amplification of heterogeneous flow. Measurements of corner frequency and resonant frequency should be considered during high-frequency oscillatory ventilation management.
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Bhatawadekar SA, Leary D, de Lange V, Peters U, Fulton S, Hernandez P, McParland C, Maksym GN. Reactance and elastance as measures of small airways response to bronchodilator in asthma. J Appl Physiol (1985) 2019; 127:1772-1781. [PMID: 31647721 DOI: 10.1152/japplphysiol.01131.2018] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Bronchodilation alters both respiratory system resistance (Rrs) and reactance (Xrs) in asthma, but how changes in Rrs and Xrs compare, and respond differently in health and asthma, in reflecting the contributions from the large and small airways has not been assessed. We assessed reversibility using spirometry and oscillometry in healthy and asthma subjects. Using a multibranch airway-tree model with the mechanics of upper airway shunt, we compared the effects of airway dilation and small airways recruitment to explain the changes in Rrs and Xrs. Bronchodilator decreased Rrs by 23.0 (19.0)% in 18 asthma subjects and by 13.5 (19.5)% in 18 healthy subjects. Estimated respiratory system elastance (Ers) decreased by 23.2 (21.4)% in asthma, with no significant decrease in healthy subjects. With the use of the model, airway recruitment of 15% across a generation of the small airways could explain the changes in Ers in asthma with no recruitment in healthy subjects. In asthma, recruitment accounted for 40% of the changes in Rrs, with the remaining explained by airway dilation of 6.8% attributable largely to the central airways. Interestingly, the same dilation magnitude explained the changes in Rrs in healthy subjects. Shunt only affected Rrs of the model. Ers was unaltered in health and unaffected by shunt in both groups. In asthma, Ers changed comparably to Rrs and could be attributed to small airways, while the change in Rrs was split between large and small airways. This implies that in asthma Ers sensed through Xrs may be a more effective measure of small airways obstruction and recruitment than Rrs.NEW & NOTEWORTHY This is the first study to quantify to relative contributions of small and large airways to bronchodilator response in healthy subjects and patients with asthma. The response of the central airways to bronchodilator was similar in magnitude in both study groups, whereas the response of the small airways was significant among patients with asthma. These results suggest that low-frequency reactance and derived elastance are both sensitive measures of small airway function in asthma.
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Affiliation(s)
- S A Bhatawadekar
- School of Biomedical Engineering, Dalhousie University, Halifax, Nova Scotia, Canada.,Department of Medicine, University of Vermont College of Medicine, Burlington, Vermont
| | - D Leary
- Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado
| | - V de Lange
- School of Biomedical Engineering, Dalhousie University, Halifax, Nova Scotia, Canada
| | - U Peters
- Department of Medicine, University of Vermont College of Medicine, Burlington, Vermont
| | - S Fulton
- Division of Respirology, QE-II Health Sciences Centre, Halifax, Nova Scotia, Canada
| | - P Hernandez
- Division of Respirology, QE-II Health Sciences Centre, Halifax, Nova Scotia, Canada.,Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
| | - C McParland
- Division of Respirology, QE-II Health Sciences Centre, Halifax, Nova Scotia, Canada.,Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
| | - G N Maksym
- School of Biomedical Engineering, Dalhousie University, Halifax, Nova Scotia, Canada
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Herrmann J, Tawhai MH, Kaczka DW. Strain, strain rate, and mechanical power: An optimization comparison for oscillatory ventilation. INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING 2019; 35:e3238. [PMID: 31318162 PMCID: PMC6785367 DOI: 10.1002/cnm.3238] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/30/2019] [Revised: 06/07/2019] [Accepted: 07/14/2019] [Indexed: 06/10/2023]
Abstract
The purpose of this study was to assess the potential for optimization of mechanical ventilator waveforms using multiple frequencies of oscillatory flow delivered simultaneously to minimize the risk of ventilator-induced lung injury (VILI) associated with regional strain, strain rate, and mechanical power. Optimization was performed using simulations of distributed oscillatory flow and gas transport in a computational model of anatomically derived branching airway segments and viscoelastic terminal acini under healthy and injured conditions. Objective functions defined by regional strain or strain rate were minimized by single-frequency ventilation waveforms using the highest or lowest frequencies available, respectively. However, a mechanical power objective function was minimized by a combination of multiple frequencies delivered simultaneously. This simulation study thus demonstrates the potential for multifrequency oscillatory ventilation to reduce regional mechanical power in comparison to single-frequency ventilation, and thereby reduce the risk of VILI.
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Affiliation(s)
- Jacob Herrmann
- Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa, USA
- Department of Anesthesia, University of Iowa, Iowa City, Iowa, USA
| | - Merryn H. Tawhai
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - David W. Kaczka
- Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa, USA
- Department of Anesthesia, University of Iowa, Iowa City, Iowa, USA
- Department of Radiology, University of Iowa, Iowa City, Iowa, USA
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Herrmann J, Tawhai MH, Kaczka DW. Computational Modeling of Primary Blast Lung Injury: Implications for Ventilator Management. Mil Med 2019; 184:273-281. [PMID: 30901433 DOI: 10.1093/milmed/usy305] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2018] [Revised: 09/27/2018] [Accepted: 10/18/2018] [Indexed: 01/02/2023] Open
Abstract
Primary blast lung injury (PBLI) caused by exposure to high-intensity pressure waves is associated with parenchymal tissue injury and severe ventilation-perfusion mismatch. Although supportive ventilation is often required in patients with PBLI, maldistribution of gas flow in mechanically heterogeneous lungs may lead to further injury due to increased parenchymal strain and strain rate, which are difficult to predict in vivo. In this study, we developed a computational lung model with mechanical properties consistent with healthy and PBLI conditions. PBLI conditions were simulated with bilateral derecruitment and increased perihilar tissue stiffness. As a result of these tissue abnormalities, airway flow was heterogeneously distributed in the model under PBLI conditions, during both conventional mechanical ventilation (CMV) and high-frequency oscillatory ventilation. PBLI conditions resulted in over three-fold higher parenchymal strains compared to the healthy condition during CMV, with flow distributed according to regional tissue stiffness. During high-frequency oscillatory ventilation, flow distribution became increasingly heterogeneous and frequency-dependent. We conclude that the distribution and rate of parenchymal distension during mechanical ventilation depend on PBLI severity as well as ventilatory modality. These simulations may allow realistic assessment of the risks associated with ventilator-induced lung injury following PBLI, and facilitate the development of alternative lung-protective ventilation modalities.
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Affiliation(s)
- Jacob Herrmann
- Department of Anesthesia, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, Iowa City, IA.,Department of Biomedical Engineering, University of Iowa, 5601 Seamans Center for the Engineering Arts and Sciences, Iowa City, IA
| | - Merryn H Tawhai
- Auckland Bioengineering Institute, University of Auckland, 6/70 Symonds St, Grafton, Auckland 1010, New Zealand
| | - David W Kaczka
- Department of Anesthesia, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, Iowa City, IA.,Department of Biomedical Engineering, University of Iowa, 5601 Seamans Center for the Engineering Arts and Sciences, Iowa City, IA.,Department of Radiology, University of Iowa Hospitals and Clinics, 3970 John Pappajohn Pavilion, 200 Hawkins Dr, Iowa City, IA
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Herrmann J, Tawhai MH, Kaczka DW. Parenchymal strain heterogeneity during oscillatory ventilation: why two frequencies are better than one. J Appl Physiol (1985) 2017; 124:653-663. [PMID: 29051332 DOI: 10.1152/japplphysiol.00615.2017] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
High-frequency oscillatory ventilation (HFOV) relies on low tidal volumes cycled at supraphysiological rates, producing fundamentally different mechanisms for gas transport and exchange compared with conventional mechanical ventilation. Despite the appeal of using low tidal volumes to mitigate the risks of ventilator-induced lung injury, HFOV has not improved mortality for most clinical indications. This may be due to nonuniform and frequency-dependent distribution of flow throughout the lung. The goal of this study was to compare parenchymal strain heterogeneity during eucapnic HFOV when using oscillatory waveforms that consisted of either a single discrete frequency or two simultaneous frequencies. We utilized a three-dimensional, anatomically structured canine lung model for simulating frequency-dependent ventilation distribution. Gas transport was simulated via direct alveolar ventilation, advective mixing at bifurcations, turbulent and oscillatory dispersion, and molecular diffusion. Volume amplitudes at each oscillatory frequency were iteratively optimized to attain eucapnia. Ventilation using single-frequency HFOV demonstrated increasing heterogeneity of acinar flow and CO2 elimination with frequency for frequencies greater than the resonant frequency. For certain pairs of frequencies, a linear combination of the two corresponding ventilation distributions yielded reduced acinar strain heterogeneity compared with either frequency alone. Our model demonstrates that superposition of two simultaneous oscillatory frequencies can achieve more uniform ventilation distribution, and therefore lessen the potential for ventilator-induced lung injury, compared with traditional single-frequency HFOV. NEW & NOTEWORTHY In this study, we simulated oscillatory ventilation with multiple simultaneous frequencies using a computational lung model that includes distributed flow and gas transport. A mechanism of benefit was identified by which ventilation with two simultaneous frequencies results in reduced acinar strain heterogeneity compared with either frequency alone. This finding suggests the possibility of tuning the spectral content of ventilator waveforms according to patient-specific mechanical heterogeneity.
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Affiliation(s)
- Jacob Herrmann
- Department of Anesthesia, University of Iowa , Iowa City, Iowa.,Department of Biomedical Engineering, University of Iowa , Iowa City, Iowa
| | - Merryn H Tawhai
- Auckland Bioengineering Institute, University of Auckland , Auckland , New Zealand
| | - David W Kaczka
- Department of Anesthesia, University of Iowa , Iowa City, Iowa.,Department of Biomedical Engineering, University of Iowa , Iowa City, Iowa.,Department of Radiology, University of Iowa , Iowa City, Iowa
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Abstract
OBJECTIVE Lungs behave as viscoelastic polymers. Harms of mechanical ventilation could then depend on not only amplitude (strain) but also velocity (strain rate) of lung deformation. Herein, we tested this hypothesis. DESIGN Laboratory investigation. SETTING Animal unit. SUBJECTS Thirty healthy piglets. INTERVENTIONS Two groups of animals were ventilated for 54 hours with matched lung strains (ratio between tidal volume and functional residual capacity) but different lung strain rates (ratio between strain and inspiratory time). Individual strains ranged between 0.6 and 3.5 in both groups. Piglets ventilated with low strain rates had an inspiratory-to-expiratory time ratio of 1:2-1:3. Those ventilated with high strain rates had much lower inspiratory-to-expiratory time ratios (down to 1:9). Respiratory rate was always 15 breaths/min. Lung viscoelastic behavior, with ventilator setting required per protocol, was "quantified" as dynamic respiratory system hysteresis (pressure-volume loop [in Joules]) and stress relaxation (airway pressure drop during an end-inspiratory pause [in cm H2O]). Primary outcome was the occurrence of pulmonary edema within 54 hours. MEASUREMENTS AND MAIN RESULTS On average, the two study groups were ventilated with well-matched strains (2.1 ± 0.9 vs 2.1 ± 0.9; p = 0.864) but different strain rates (1.8 ± 0.8 vs 4.6 ± 1.5 s; p < 0.001), dynamic respiratory system hysteresis (0.6 ± 0.3 vs 1.4 ± 0.8 J; p = 0.001), and stress relaxation (3.1 ± 0.9 vs 5.0 ± 2.3 cm H2O; p = 0.008). The prevalence of pulmonary edema was 20% among piglets ventilated with low strain rates and 73% among those ventilated with high strain rates (p = 0.010). CONCLUSIONS High strain rate is a risk factor for ventilator-induced pulmonary edema, possibly because it amplifies lung viscoelastic behavior.
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Herrmann J, Tawhai MH, Kaczka DW. Regional gas transport in the heterogeneous lung during oscillatory ventilation. J Appl Physiol (1985) 2016; 121:1306-1318. [PMID: 27763872 PMCID: PMC5206385 DOI: 10.1152/japplphysiol.00097.2016] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2016] [Revised: 09/29/2016] [Accepted: 09/29/2016] [Indexed: 11/22/2022] Open
Abstract
Regional ventilation in the injured lung is heterogeneous and frequency dependent, making it difficult to predict how an oscillatory flow waveform at a specified frequency will be distributed throughout the periphery. To predict the impact of mechanical heterogeneity on regional ventilation distribution and gas transport, we developed a computational model of distributed gas flow and CO2 elimination during oscillatory ventilation from 0.1 to 30 Hz. The model consists of a three-dimensional airway network of a canine lung, with heterogeneous parenchymal tissues to mimic effects of gravity and injury. Model CO2 elimination during single frequency oscillation was validated against previously published experimental data (Venegas JG, Hales CA, Strieder DJ, J Appl Physiol 60: 1025-1030, 1986). Simulations of gas transport demonstrated a critical transition in flow distribution at the resonant frequency, where the reactive components of mechanical impedance due to airway inertia and parenchymal elastance were equal. For frequencies above resonance, the distribution of ventilation became spatially clustered and frequency dependent. These results highlight the importance of oscillatory frequency in managing the regional distribution of ventilation and gas exchange in the heterogeneous lung.
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Affiliation(s)
- Jacob Herrmann
- Department of Anesthesia, University of Iowa, Iowa City, Iowa.,Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa
| | - Merryn H Tawhai
- Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand; and
| | - David W Kaczka
- Department of Anesthesia, University of Iowa, Iowa City, Iowa; .,Department of Biomedical Engineering, University of Iowa, Iowa City, Iowa.,Department of Radiology, University of Iowa, Iowa City, Iowa
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Amini R, Herrmann J, Kaczka DW. Intratidal Overdistention and Derecruitment in the Injured Lung: A Simulation Study. IEEE Trans Biomed Eng 2016; 64:681-689. [PMID: 27244715 DOI: 10.1109/tbme.2016.2572678] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
GOAL Ventilated patients with the acute respiratory distress syndrome (ARDS) are predisposed to cyclic parenchymal overdistention and derecruitment, which may worsen existing injury. We hypothesized that intratidal variations in global mechanics, as assessed at the airway opening, would reflect such distributed processes. METHODS We developed a computational lung model for determining local instantaneous pressure distributions and mechanical impedances continuously during a breath. Based on these distributions and previous literature, we simulated the within-breath variability of airway segment dimensions, parenchymal viscoelasticity, and acinar recruitment in an injured canine lung for tidal volumes( VT ) of 10, 15, and 20 mL·kg-1 and positive end-expiratory pressures (PEEP) of 5, 10, and 15 cm H2O. Acini were allowed to transition between recruited and derecruited states when exposed to stochastically determined critical opening and closing pressures, respectively. RESULTS For conditions of low VT and low PEEP, we observed small intratidal variations in global resistance and elastance, with a small number of cyclically recruited acini. However, with higher VT and PEEP, larger variations in resistance and elastance were observed, and the majority of acini remained open throughout the breath. Changes in intratidal resistance, elastance, and impedance followed well-defined parabolic trajectories with tracheal pressure, achieving minima near 12 to 16 cm H2O. CONCLUSION Intratidal variations in lung mechanics may allow for optimization of ventilator settings in patients with ARDS, by balancing lung recruitment against parenchymal overdistention. SIGNIFICANCE Titration of airway pressures based on variations in intratidal mechanics may mitigate processes associated with injurious ventilation.
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Multifrequency Oscillatory Ventilation in the Premature Lung: Effects on Gas Exchange, Mechanics, and Ventilation Distribution. Anesthesiology 2016; 123:1394-403. [PMID: 26495977 DOI: 10.1097/aln.0000000000000898] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
BACKGROUND Despite the theoretical benefits of high-frequency oscillatory ventilation (HFOV) in preterm infants, systematic reviews of randomized clinical trials do not confirm improved outcomes. The authors hypothesized that oscillating a premature lung with multiple frequencies simultaneously would improve gas exchange compared with traditional single-frequency oscillatory ventilation (SFOV). The goal of this study was to develop a novel method for HFOV, termed "multifrequency oscillatory ventilation" (MFOV), which relies on a broadband flow waveform more suitable for the heterogeneous mechanics of the immature lung. METHODS Thirteen intubated preterm lambs were randomly assigned to either SFOV or MFOV for 1 h, followed by crossover to the alternative regimen for 1 h. The SFOV waveform consisted of a pure sinusoidal flow at 5 Hz, whereas the customized MFOV waveform consisted of a 5-Hz fundamental with additional energy at 10 and 15 Hz. Per standardized protocol, mean pressure at airway opening ((Equation is included in full-text article.)) and inspired oxygen fraction were adjusted as needed, and root mean square of the delivered oscillatory volume waveform (Vrms) was adjusted at 15-min intervals. A ventilatory cost function for SFOV and MFOV was defined as (Equation is included in full-text article.), where Wt denotes body weight. RESULTS Averaged over all time points, MFOV resulted in significantly lower VC (246.9 ± 6.0 vs. 363.5 ± 15.9 ml mmHg kg) and (Equation is included in full-text article.)(12.8 ± 0.3 vs. 14.1 ± 0.5 cm H2O) compared with SFOV, suggesting more efficient gas exchange and enhanced lung recruitment at lower mean airway pressures. CONCLUSION Oscillation with simultaneous multiple frequencies may be a more efficient ventilator modality in premature lungs compared with traditional single-frequency HFOV.
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Leary D, Winkler T, Braune A, Maksym GN. Effects of airway tree asymmetry on the emergence and spatial persistence of ventilation defects. J Appl Physiol (1985) 2014; 117:353-62. [PMID: 24947031 DOI: 10.1152/japplphysiol.00881.2013] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Asymmetry and heterogeneity in the branching of the human bronchial tree are well documented, but their effects on bronchoconstriction and ventilation distribution in asthma are unclear. In a series of seminal studies, Venegas et al. have shown that bronchoconstriction may lead to self-organized patterns of patchy ventilation in a computational model that could explain areas of poor ventilation [ventilation defects (VDefs)] observed in positron emission tomography images during induced bronchoconstriction. To investigate effects of anatomic asymmetry on the emergence of VDefs we used the symmetric tree computational model that Venegas and Winkler developed using different trees, including an anatomic human airway tree provided by M. Tawhai (University of Auckland), a symmetric tree, and three trees with intermediate asymmetry (Venegas JG, Winkler T, Musch G, Vidal Melo MF, Layfield D, Tgavalekos N, Fischman AJ, Callahan RJ, Bellani G, Harris RS. Nature 434: 777-782, 2005 and Winkler T, Venegas JG. J Appl Physiol 103: 655-663, 2007). Ventilation patterns, lung resistance (RL), lung elastance (EL), and the entropy of the ventilation distribution were compared at different levels of airway smooth muscle activation. We found VDefs emerging in both symmetric and asymmetric trees, but VDef locations were largely persistent in asymmetric trees, and bronchoconstriction reached steady state sooner than in a symmetric tree. Interestingly, bronchoconstriction in the asymmetric tree resulted in lower RL (∼%50) and greater EL (∼%25). We found that VDefs were universally caused by airway instability, but asymmetry in airway branching led to local triggers for the self-organized patchiness in ventilation and resulted in persistent locations of VDefs. These findings help to explain the emergence and the persistence in location of VDefs found in imaging studies.
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Affiliation(s)
- D Leary
- Dalhousie University, Halifax, Nova Scotia, Canada; and
| | - T Winkler
- Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts
| | - A Braune
- Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts
| | - G N Maksym
- Dalhousie University, Halifax, Nova Scotia, Canada; and
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Amini R, Kaczka DW. Impact of ventilation frequency and parenchymal stiffness on flow and pressure distribution in a canine lung model. Ann Biomed Eng 2013; 41:2699-711. [PMID: 23872936 DOI: 10.1007/s10439-013-0866-7] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2013] [Accepted: 07/08/2013] [Indexed: 11/25/2022]
Abstract
To determine the impact of ventilation frequency, lung volume, and parenchymal stiffness on ventilation distribution, we developed an anatomically-based computational model of the canine lung. Each lobe of the model consists of an asymmetric branching airway network subtended by terminal, viscoelastic acinar units. The model allows for empiric dependencies of airway segment dimensions and parenchymal stiffness on transpulmonary pressure. We simulated the effects of lung volume and parenchymal recoil on global lung impedance and ventilation distribution from 0.1 to 100 Hz, with mean transpulmonary pressures from 5 to 25 cm H2O. With increasing lung volume, the distribution of acinar flows narrowed and became more synchronous for frequencies below resonance. At higher frequencies, large variations in acinar flow were observed. Maximum acinar flow occurred at first antiresonance frequency, where lung impedance achieved a local maximum. The distribution of acinar pressures became very heterogeneous and amplified relative to tracheal pressure at the resonant frequency. These data demonstrate the important interaction between frequency and lung tissue stiffness on the distribution of acinar flows and pressures. These simulations provide useful information for the optimization of frequency, lung volume, and mean airway pressure during conventional ventilation or high frequency oscillation (HFOV). Moreover our model indicates that an optimal HFOV bandwidth exists between the resonant and antiresonant frequencies, for which interregional gas mixing is maximized.
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Lin CL, Tawhai MH, Hoffman EA. Multiscale image-based modeling and simulation of gas flow and particle transport in the human lungs. WILEY INTERDISCIPLINARY REVIEWS-SYSTEMS BIOLOGY AND MEDICINE 2013; 5:643-55. [PMID: 23843310 DOI: 10.1002/wsbm.1234] [Citation(s) in RCA: 44] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/16/2012] [Revised: 05/20/2013] [Accepted: 05/30/2013] [Indexed: 12/21/2022]
Abstract
Improved understanding of structure and function relationships in the human lungs in individuals and subpopulations is fundamentally important to the future of pulmonary medicine. Image-based measures of the lungs can provide sensitive indicators of localized features, however to provide a better prediction of lung response to disease, treatment, and environment, it is desirable to integrate quantifiable regional features from imaging with associated value-added high-level modeling. With this objective in mind, recent advances in computational fluid dynamics (CFD) of the bronchial airways-from a single bifurcation symmetric model to a multiscale image-based subject-specific lung model-will be reviewed. The interaction of CFD models with local parenchymal tissue expansion-assessed by image registration-allows new understanding of the interplay between environment, hot spots where inhaled aerosols could accumulate, and inflammation. To bridge ventilation function with image-derived central airway structure in CFD, an airway geometrical modeling method that spans from the model 'entrance' to the terminal bronchioles will be introduced. Finally, the effects of turbulent flows and CFD turbulence models on aerosol transport and deposition will be discussed.
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Affiliation(s)
- Ching-Long Lin
- Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA, USA
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Simon BA, Kaczka DW, Bankier AA, Parraga G. What can computed tomography and magnetic resonance imaging tell us about ventilation? J Appl Physiol (1985) 2012; 113:647-57. [PMID: 22653989 DOI: 10.1152/japplphysiol.00353.2012] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
This review provides a summary of pulmonary functional imaging approaches for determining pulmonary ventilation, with a specific focus on multi-detector x-ray computed tomography and magnetic resonance imaging (MRI). We provide the important functional definitions of pulmonary ventilation typically used in medicine and physiology and discuss the fact that some of the imaging literature describes gas distribution abnormalities in pulmonary disease that may or may not be related to the physiological definition or clinical interpretation of ventilation. We also review the current state-of-the-field in terms of the key physiological questions yet unanswered related to ventilation and gas distribution in lung disease. Current and emerging imaging research methods are described, including their strengths and the challenges that remain to translate these methods to more wide-spread research and clinical use. We also examine how computed tomography and MRI might be used in the future to gain more insight into gas distribution and ventilation abnormalities in pulmonary disease.
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Affiliation(s)
- Brett A Simon
- Department of Anesthesia, Critical Care and Pain Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA
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